The weldability of aluminum alloys (i.e., the base alloy or material) can be defined as the alloy's resistance to hot tearing during weld solidification. The primary factors that render aluminum alloys more susceptible to developing hot tears during welding relative to other metallic alloy systems are the relatively high thermal expansion coefficient and solidification shrinkage of aluminum. These factors are further compounded when one or more alloying elements are added to aluminum to achieve technologically useful engineering alloys with improved properties (e.g., strength and elongation). More specifically, unlike pure aluminum, which has a definite melting temperature, two-component or multi-component aluminum alloys solidify over a wide temperature interval between the liquidus and solidus temperatures. A large solidification range allows more time for the deleterious thermal expansion and volumetric changes to generate sufficient stresses that ultimately cause tearing of the liquid films that partition to interdendritic sites.
Many high strength aluminum alloys have been developed and are generally categorized according to the primary alloying addition (e.g., Al-Cu:2XXX; A1-Mg:5XXX; A1-Si:6XXX; and Al -Zn:7XXX). Since there is a single primary alloying element, these alloys are commonly referred to as binary systems. However, certain ancillary alloying additions are often included to produce a wide range of alloys that are targeted for several end use applications. It is standard practice, for example, to add grain refining elements, such as Ti, Zr, Cr, Mn, V, Yt, Nb, B, TiB.sub.2 and Hf, to further improve the processing characteristics and properties of these alloy systems. Due to the enhanced properties of these types of alloys, it would be desirable to use these types of alloys in structures which are preferably assembled via welding.
The weldability of high strength aluminum alloys is dependent at least in part on the amount of the alloying elements in the base material. The general behavior of binary alloy systems in welding applications can be divided into three categories: very low alloying levels, high alloying levels approaching the solid solubility limit in aluminum, and intermediate alloying levels. At very low alloying levels approaching pure aluminum, cracking during solidification is very low since dendrites tend to interlock with virtually formation of an interdendritic liquid film. At high alloying levels, relatively low cracking is also observed. Even though there is a relatively large solidification range with the formation of interdendritic liquid films, any hot tearing that occurs during solidification is healed by the backfilling of the last-to-solidify eutectic liquid. This is particularly evident in the binary alloys that form eutectic phases (e.g., Cu, Mg and Si). It is the intermediate alloy levels that are most susceptible to hot tearing. Although a eutectic liquid film partitions into interdendritic sites, the thermal contraction of the dendrites induces sufficient strain to tear these liquid films, thereby resulting in the presence of cracks in the solidified weldment. In contrast to the highly alloyed binary alloys, there is an insufficient amount of eutectic liquid available to mend hot tears. An exception to these trends was reported for Zn additions. Since there is no eutectic phase in the Al-Zn system, hot tearing susceptibility continuously increases as Zn content is increased.
In addition to the amount of alloying element having an effect on weldability of binary systems, the type of ternary alloying elements plays a key role in affecting weldability. For instance, many strength-increasing additions to binary alloy systems have deleterious effects on weldability. For example, small additions of Mg to Al-Cu (i.e., 2XXX alloys) significantly improves the alloy strength. One example, alloy 2024 (Al-4.3 Cu-1.5 Mg-0.60 Mn) is widely used in aircraft construction. Since the Mg additions greatly increase the melting range, however, weldability is severely compromised. Consequently, alloy 2024 is typically not used in welded structures. The highest strength alloys are the Al-Zn-Mg (i.e., 7XXX system), particularly those with Cu additions. The additions of Cu can increase the solidification range by as much as 100.degree. C., generally resulting in poor weldability. Thus, despite the promising properties of these alloys, they are rarely used in situations which require adequate weldability.
Given the current limitations with regard to the use of the high strength aluminum alloys in welded structures, it would be highly desirable to redesign these alloys to enhance weldability while either maintaining or increasing mechanical properties, such as strength and elongation.
Another important component in welding aluminum alloys is the filler wire. With most welding processes, an initial penetration pass with the welding torch causes displacement of the molten metal into the opposite side of the plate. It is necessary to compensate for this displacement by continuous feeding of a filler alloy into the weldment either during the initial penetration pass or in a number of subsequent multiple passes. The resulting weldment is then a mixture of the original base alloy and the filler alloy with the ratio of the filler alloy and base alloy mixture being dependent upon the joint geometry. For example, a "V-joint" geometry is typically employed when welding relatively thick aluminum plate and contains a proportionally high amount of filler alloy (e.g., 70%-90%). At the other end of the spectrum is the butt joint geometry that is used for relatively thin gauge weldments, resulting in a relatively low filler alloy content (e.g., 10%-30%).
Modern references to weldability indicate that filler alloy selection can greatly influence hot tearing resistance, particularly at high dilution levels (i.e., high filler alloy content). By examining a listing of aluminum filler alloy compositions, it can be observed that most filler alloys contain a high level of one solute (e.g., Cu, Si, or Mg) and grain refining elements (e.g., Mn, Cr, Ti, Zr, V, Yt, Nb, B, TiB.sub.2 and Hf). Since these alloys are designed only for welding purposes, it is typically a filler alloy design constraint that only one major alloying addition can be made to minimize the solidification range. Accordingly, filler alloys rarely obtain the properties of complex wrought aluminum alloys such as 2024, 7075 and 6061. Further when a filler alloy is deposited, the weld microstructure is similar to the lowest strength, as-cast condition, further resulting in low strength properties. The combination of limiting filler alloy compositions to one primary alloying addition and the fact that strength properties are in accordance with the as-cast condition results in weldment yield strength properties that are as low as one-third that of the base alloy. Accordingly, a design that involves a welded plate is often three times thicker than the non-welded portions of the structure, resulting in a severe weight penalty. In weight-critical aerospace structures, this design constraint is overcome by using a thick plate in the areas to be welded and chemically milling the remaining areas. This approach can somewhat alleviate the weight penalty, but can create other problems such as additional material cost, added processing cost, and adverse effects on the environment by converting the majority of the aluminum plate to toxic chemical waste.
It would be highly desirable to offer designers of welded structures an improved approach for fabricating aluminum structures. Such approach could involve modifications to both the aluminum base alloy and the aluminum filler alloy.